This invention relates to a process for catalytic reforming utilizing a highly active catalyst, particularly a calcium promoted, nickel catalyst on an alumina support. This catalyst is particularly useful for this process as it is highly active and resistant to coking especially in a feed stream containing relatively high quantities of sulfur compounds, significant quantities of CO and CO2, and relatively low quantities of steam (the feed stream having a H2O/CH4<0.8 and a CO2/CH4>0.5).
Production of synthesis gas or syngas (various blends of gases generally comprising hydrogen and carbon monoxide) is an important process step in the manufacture of numerous chemicals, such as ammonia and methanol. It is also useful in numerous other commercial processes, such as iron ore reduction, Fischer-Tropsch synthesis and gas-to-liquid technology. Many of the synthesis gas plants produce the syngas by steam reforming hydrocarbons. Typically, these plants employ a process of catalytic steam reforming of methane in the presence of a supported nickel catalyst, usually a nickel on alumina support or promoted alumina support catalyst, which process is described by the following reaction:CH4+H2O⇄CO+3H2
For a variety of reasons including the prevention of coking, an excess quantity of steam is generally required to be present in the feed stream for this reaction. This excess of steam often results in a water gas shift reaction, namelyCO+H2O⇄CO2+H2occurring at the same time as the steam reforming reaction. As a result of these two reactions occurring at the same time, the syngas stream created typically has an H2:CO ratio greater than 3. (A ratio of three is predicted by the steam reforming reaction alone.) This high concentration of hydrogen in the reaction product is desirable if hydrogen is the targeted product, such as for NH3 synthesis. However, a different H2:CO ratio is required if the syngas is to be used for other processes, such as iron ore reduction, the production of methanol, Fischer-Tropsch synthesis or gas-to-liquid technology. These processes typically require a H2:CO ratio in the feed stream in the range of 1.4–2.4, and control of the relative quantities of H2 and CO present in the feed stream is important to these processes.
Because syngas with an H2:CO ratio of about 2 cannot be produced by a conventional steam reforming reaction, mixed reforming reactions are required where part of the steam in the feed stream is replaced with carbon dioxide. One use of the reaction feed from this type of reaction is for iron ore reduction. In this reaction a reducing gas, comprising mainly H2 and CO and including nominal amounts of CO2 and H2O, is generated in a reformer. This stream is then fed into a furnace where the iron ore is reduced to iron by the following reactions:3CO+Fe2O3→2Fe+3CO23H2+Fe2O3→2Fe+3H2OThe gas effluents from the reduction furnace, that contain lower percentages of H2 and CO and higher percentages of H2O and CO2 plus residual CH4, are then saturated with H2O in a boiler and recycled back to the reformer as feed stock along with additional hydrocarbons, usually natural gas or CH4.
CO and H2 are necessary components of the iron ore reduction process while H2O and CO2 function as oxidants and, therefore, are undesirable components which need to be minimized. Ideally the quantity of H2O+CO2 present in the reformed gas is limited to less than 5 percent. If excess steam and carbon dioxide are present in a feed stream or the reformed gas stream for iron ore reduction, the reducing capability of the reformed gas is minimized, sometimes substantially. Cooling the reformed gas before it enters the reduction furnace can remove some H2O, but cooling results in low energy efficiency of the entire process and is not economically feasible.
In iron ore reduction applications, a typical feed stream for the reformers comprises about 10%–15% H2O, 10%–18% CO2, 15%–20% CO, 15%–25% hydrocarbons (usually natural gas) and the balance H2. (Unless otherwise noted, all the percentages are mole base.) In contrast, in conventional steam reforming reactions, the quantity of H2O in the feed stream may be as high as 86 percent and is normally at least about 66 percent. The balance of the steam reformer feed stream comprises hydrocarbons (usually natural gas). Minor amounts of CO2 may be present with the natural gas.
In order to maximize H2 and CO concentrations and minimize H2O and CO2 concentrations in the reformed gas, in addition to using low quantities of H2O, the outlet temperature of the reformer needs to be maintained as high as possible with the temperature usually limited only by the composition of the metal of the reformer tubes. The outlet temperature of this type of reformer is usually maintained in a range of 850° C. to 1000° C., which is higher than that at the outlet of a conventional steam reformer (700° C. to 800° C.). This outlet temperature range is another significant distinction between conventional steam reforming and the reforming utilized for iron ore reduction.
The presence of sulfur compounds in the reforming system deactivates conventional steam reforming catalysts. In fact, quantities of H2S as low as one part per billion substantially deactivate many conventional steam reforming catalysts. Therefore, sulfur is usually removed before being allowed to enter the reformer. In contrast, in the reducing gas generation process, the feed may include a significant amount of sulfur or sulfur compounds, and the catalysts need to retain sufficiently high reforming activity even at sulfur levels up to about 20 parts per million. While higher temperatures and H2 partial pressures in the reaction feed can reduce the level of deactivation in these feed streams containing sulfur or sulfur compounds, these higher temperatures may also adversely affect the physical structure of the steam reforming catalysts.
Another problem that often occurs with reforming reactions is an enhanced likelihood of coking or carbon formation on the catalysts. In conventional reforming processes, there is essentially no CO in the feed stream. In contrast, in reducing gas generation processes, the low H2O and high CO and CO2 conditions make coking of the reforming catalysts a problem. In this situation, carbon formation caused by CO at the inlet section of the reformer in the reducing gas generation process occurs. On nickel catalysts the effect of this coking is coating of the active nickel sites and plugging of the pores of the catalyst. In one solution to this problem, manufacturers have used a large excess of H2O in the reformer feed stream, but this is not suitable for reducing gas generation processes.
Another method of solving the coking problem is by use of a noble metal catalyst, such as is disclosed in U.S. Pat. No. 5,753,143. It is well known that noble metal catalysts have higher coke formation resistance compared to conventional steam reforming catalysts, which merely utilize nickel as the active component. However, these noble metal catalysts are quite expensive, especially with the large quantity of catalysts that is conventionally utilized for this type of reaction.
Another recognized method of addressing the coking problem is by the use of a high dispersion of metal species over the surface of the catalyst, such as the various types of double hydroxide catalysts which are disclosed by Morioka, H., et al. “Partial oxidation of methane to synthesis gas over supported Ni catalysts prepared from Ni—Ca/Al-layered double hydroxide,” Applied Catalysis A: General 215 pages 11–19, (2001).
Another proposed solution to this coking problem is disclosed in U.S. Pat. No. 4,530,918 which teaches a nickel on alumina catalyst with a lanthanum additive.
Another process for limiting coke formation on nickel catalysts during reforming reactions utilizes the sulfur that is naturally present in the feed stream. In this process sulfur poisons some, but not all, of the nickel sites on the catalyst and produces a reforming catalyst which retains sufficient active sites to be useful for gas production at lower H2/CO ratios. The mechanism of preventing carbon formation by sulfur poisoning, or passivation, has been described in Udengaard, Niels R., et al. “Sulfur passivated reforming process lowers syngas H2/CO ratio.” Oil and Gas Journal, 62–67 (1992). Obviously, in using this process it is critical to control the amount of sulfur that is present in the feed stream so that the catalyst retains sufficient activity for the reforming reaction. This reaction often requires the presence of a substantial quantity of catalyst in the bed.
A method for steam reforming hydrocarbons containing sulfur compounds utilizing a catalyst comprising a noble metal catalyst, a support phase and optionally a catalyst promoter is disclosed in U.S. Pat. No. 6,238,816.
Additives are often added to these conventional steam reforming nickel on alumina catalysts to enhance their performance and to reduce the coking problem. For example, alkali compounds are added to steam reforming catalysts to reduce carbon formation in Trimm, O. L.; Applied Catalysis, 5, 263 (1983), Borowiecki, T.; Applied Catalysis, 4, 223 (1982), and Tottrup, P. and Nielson, R.; Hydrocarbon Processing, 89 (March 1982). However, there are drawbacks to the use of alkali metals because of their potential migration during high temperature processing, which can adversely impact downstream operations.
Magnesia has also been added to steam reforming catalysts to suppress carbon formation, as disclosed in Trimm, O. L.; Applied Catalysis, 5, 263 (1983) and Borowiecki, T.; Applied Catalysis, 4, 223 (1982). However, one major drawback to the use of magnesia promoted catalysts is that they are hard to reduce and maintain in a reduced state as reported by Parmaliana et al.; “Structure and Reactivity of Surfaces” p 739, (1989). Nickel oxide and magnesia are very similar in structure. Thus, a nickel oxide and magnesia combination material is usually formed during the high temperature reaction. The reducibility of nickel oxide and the activity of a magnesia-based catalyst is heavily dependent on the calcination temperature, as mentioned in Takezawa, et al. Applied Catalysis, 23, 291 (1986). A calcination temperature higher than 400C results in less active catalysts as discussed in Parmaliana, et al.; “Structure and Reactivity of Surfaces” p. 739 (1989). In order to form an effective catalyst, the magnesia must totally combine with the alumina support to form magnesium aluminate. If free magnesia is present, it can be hydrated on stream and react with the carbon dioxide during the reforming reaction, resulting in physical degradation of the catalyst. Thus, magnesia-supported nickel catalysts are difficult to utilize for reforming reactions, especially for reducing gas generation.
A nickel catalyst for reducing gas generation is conventionally produced by impregnating nickel on an alumina or magnesia carrier. In use because the reforming reaction is a strongly endothermic reaction and in order to obtain high hydrocarbon conversion, high temperatures are required for the reaction, sometimes running as high as 1000° C. Even when the reaction is conducted at lower temperatures, in the range of 700° C., it is still necessary to use low surface area alumina, such as alpha alumina as the carrier material for these catalysts. In fact, alpha alumina is the only alumina phase that is stable enough to be used as a carrier under conventional reforming conditions. With catalysts produced from alpha alumina carriers, however, the BET surface area, pore volume and nickel dispersion on these catalysts is quite low. For example, a conventional steam reforming catalyst of this type prepared with nickel on alpha alumina has a BET surface area in the range of 1–4 m2/g, a pore volume from about 0.08 to 0.16 cc/gm and a nickel surface area from about 0.5 to 1.5 m2/g.
While lanthanum-promoted alumina catalysts of U.S. Pat. No. 4,530,918 have shown some advantages in the production of carbon monoxide rich syngas at close to stoichiometric requirements, the surface area and nickel dispersion of these catalysts is still in a range comparable to conventional alpha alumina-based steam reforming catalysts with BET surface areas only slightly improved to about 5 m2/gm with a nickel specific surface area less than 2 m2/g.
Thus, there is still a need to improve existing nickel on alumina catalysts for reforming reactions utilizing a feed stream containing significant quantities of CO and CO2 and low quantities of steam (the feed stream having a H2O/CH4<0.8 and a CO2/CH4>0.5). Further, the addition of additives to these catalysts has not to date shown sufficient satisfactory results to overcome the coking problems while maintaining high reforming activity in the presence of a significant amount of sulfur.
Therefore, it is an object of the invention to disclose a process for catalytic reforming of a feed stream containing significant quantities of CO and CO2 and low quantities of steam (the feed stream having a H2O/CH4<0.8 and a CO2/CH4>0.5) utilizing a catalyst of a particular composition.
These and other objects are obtained by the catalyst of the invention, its process of manufacture and process of use.